In order to meet the power requirements of point of use power supplies in telecommunications and computer systems, there is a trend to operate the power supplies of such telecommunication and computer systems at high switching frequencies. As the switching frequencies increase, switching losses associated with the turn-on and turn-off of the devices in the power supplies also increase. In switch mode power supplies, these losses are so significant that the operation of the power supplies at very high frequencies are prohibitive due to low conversion efficiencies.
To overcome such drawbacks, constant frequency resonant converters have been used in telecommunications equipment. Constant frequency resonant converters provide high power density, high efficiency and low switching losses while operating at a constant frequency. Constant frequency resonant converters can also be operated at high frequencies. A drawback associated with high frequency resonant converters is that they have a high component count. This high component count results in lower power densities at low to medium power levels.
Asymmetrical pulse width modulated (APWM) resonant DC--DC converters are used in conjunction with devices having low to medium power level requirements. APWM converters offer near-zero voltage switching (ZVS) losses while operating at constant and very high frequencies. APWM converters have a low component count which makes operation at low to medium power levels feasible. Drawbacks associated with known APWM converters include the fact that the input voltage range over which ZVS occurs is narrow. Also, the efficiency of APWM converters decreases as the input voltage increases. Further, resonant inductor losses increase as the voltage across the resonant inductor increase.